Multilayer-molded products of thermoplastic resins that contain a foam layer and a non-foamed layer have been used in various fields for a long time. The foam layer in a multilayer-molded product is light in weight and has excellent heat-insulating properties, sound-absorbing properties, and texture, because of bubbles inside the resin. The non-foamed layer in a multilayer-molded product can provide rigidity and excellent appearance. In recent years, in particular, resins used in products have been foamed to reduce the amount of resin for the purpose of weight reduction. Combined with the cost reduction resulting from the weight reduction, multilayer-molded products that contain a foam layer have found wider applications.
A foam layer in a multilayer-molded product is manufactured by physical foaming or chemical foaming according to the type of a foaming agent to be mixed with a resin. The physical foaming utilizes an inert gas, such as nitrogen or carbon dioxide, or a volatile substance, such as a hydrocarbon or a fluorocarbon, as a physical foaming agent. The chemical foaming utilizes an organic foaming agent, such as an azo compound or a nitroso compound, or an inorganic foaming agent, such as sodium bicarbonate, as a chemical foaming agent. A multilayer-molding method includes injecting a molten resin that contains a foaming agent and a resin material into a mold cavity, injecting a molten resin that contains a resin material and no foaming agent into the mold cavity, and foaming a resin mixed with the foaming agent. A multilayer-molded product thus manufactured includes a foam layer that contains bubbles having a diameter of about 80 to 300 μm and a non-foamed layer.
As a method for manufacturing a foam-molded product serving as a foam layer in a multilayer-molded product, for example, Patent Document 1 discloses a method for forming a foam-molded product having bubbles therein, in which an olefin resin mixed with a chemical foaming agent or a physical foaming agent is foamed by a short-shot method. According to the method disclosed in Patent Document 1, in a molding apparatus including an extruder, an accumulator, and a mold, a molten resin that contains a resin material mixed with an inert gas, such as a nitrogen gas, a volatile substance, such as a hydrocarbon or a fluorocarbon, or a chemical foaming agent is fed to the accumulator with an extruder. The molten resin fed to the accumulator is injected into the mold and is foamed, thus yielding a foam-molded product that contains bubbles therein.
Furthermore, for example, Patent Document 2 discloses a method for manufacturing a foam-molded product using a physical foaming agent, in which air, another gas, or a volatile substance is fed under pressure from an extruder hopper simultaneously with resin-material feed. The melting of the resin material and the inclusion and dispersion of bubbles are performed with a screw extruder. According to the method disclosed in Patent Document 2, a molded product of a sponge-like substance that contains closed cells can be manufactured by using a polyethylene resin and air having a pressure in the range of about 0.69 to 0.78 MPa.
Furthermore, for example, Patent Document 3 discloses a method for remarkably increasing the bubble density (the number of bubbles per unit volume) by using carbon dioxide, an inert gas, in a supercritical state as a foaming gas, as compared with bubbles formed in a molded product using a conventional chemical foaming agent or a conventional physical foaming agent. According to the method disclosed in Patent Document 3, a molding apparatus is provided with a system composed of a booster and a feeder of a supercritical fluid and a gas cylinder. Carbon dioxide in a supercritical state is injected and dissolved into a molten resin via a cylinder of the molding apparatus. The molten resin that contains dissolved carbon dioxide is injected into a mold, and is foamed, thus yielding a resin-molded product that contains ultrafine pores, called microcells, having a size less than 1 μm therein.    [Patent Document 1] JP-B-44-6080    [Patent Document 2] JP-B-43-9913    [Patent Document 3] JP-K-6-506724    [Non-patent Document 1] SEIKEI KAKOU, 2001, No. 2, Vol. 13